Clinical electrophysiological studies are important in the evaluation of patients with Guillain-Barré syndrome. Physiological evidence of demyelination occurs in almost all patients with Guillain-Barré syndrome, especially if serial studies are performed. Criteria for demyelination are proposed. Prognostic information concerning patient outcome can be deduced from computation of the mean of the summed motor evoked compound muscle action potential amplitudes from distal stimulation. Future studies of the electrophysiology of Guillain-Barré syndrome should allow increased use of the data for prognostication and should enhance our understanding of the mechanisms of the disease.